The invention relates to an actuating piston for a hydraulically and mechanically actuable, spot-type disk brake. Disk brakes of said type are usually actuated for a service braking operation hydraulically, e.g. by depressing the brake pedal of a vehicle brake system, but they may also be used as a parking brake (so-called hand brake) and for said purpose are actuated mechanically, e.g. by means of a hand brake lever or a separate parking brake pedal. In disk brakes of the described type the actuating piston, often also referred to as the brake piston, may accordingly be pressed on the one hand by means of hydraulic pressure and on the other hand by a purely mechanical device against the brake lining or linings in order to achieve the brake application force required for a braking operation or for parking.
EP 0 403 635 B1 describes such a disk brake, in which during utilization as a parking brake the brake application force is transmitted to the actuating piston via an adjusting mechanism used to compensate brake lining wear. Here, a critical element as regards load is a cone clutch, which is formed by a conical flange of a strut part of the adjusting mechanism and a complementary conical surface formed on the actuating piston. Besides axial transmission of the brake application force from the adjusting mechanism to the actuating piston, the cone clutch is also intended to prevent twisting of the said two parts relative to one another. Only in the event of a compensation of lining wear by the adjusting mechanism is a slight twisting of the clutch surfaces relative to one another to be possible.
The object of the invention is to provide an actuating piston, which is suitable for use in a brake of the type described and is lighter than conventionally used actuating pistons. In particular the actuating piston to be provided, while being of a lighter weight, is to transmit high axial forces, present a low radial expansion and be capable of withstanding the stressing of the force-transmitting conical surface resulting from frictional wear.
Proceeding from the actuating piston of a spot-type disk brake such as is known from EP 0 403 635 B1, said object is achieved according to the invention in that the part of the actuating piston comprising the axial-force-transmitting conical surface is made of steel and the part of the actuating piston cooperating with the cylinder bore of the disk brake is made of light metal or of plastics material, and that the said parts of the actuating piston take the form of a single-piece composite component. The material pairing according to the invention of steel with light metal or steel with plastics material, combined with the construction of the actuating piston as a single-piece composite component, enables trouble-free transmission of the high axial forces in the order of magnitude of two tonnes despite the significantly reduced weight of the actuating piston. The axial-force-transmitting conical surface, by virtue of being made of steel, is moreover highly resistant to stress resulting from frictional wear.
In a preferred embodiment of an actuating piston according to the invention, said actuating piston is hollow and the steel component comprising the axial-force-transmitting conical surface takes the form of a hollow-cylindrical insert accommodated in the plastic or light metal part. Along with an extremely weight-saving construction this ensures good transmission of the axial-forces by the actuating piston.
When the actuating piston according to the invention is hollow, it frequently comprises a first portion with a smaller inside diameter and a second portion with a larger inside diameter. The steel component comprising the axial-force-transmitting conical surface then extends along at least approximately the entire length of the second portion, i.e. along the portion with the larger inside diameter. The effect thereby achieved is that the part of the piston skirt, which is subjected to the greatest load by the hydraulic pressure during braking and which moreover has a lower wall thickness than the remaining part of the piston skirt, is effectively strengthened by the steel component. A radial expansion of the piston skirt in said region, which may lead to jamming of the actuating piston in the cylinder, or even a cracking-open of the piston skirt is therefore counteracted. Preferably, the steel component is tubular and is manufactured by a sheet-metal forming method such as, for example, deep drawing or pressing. Pressing, here, is taken to mean that the tubular steel component is rolled against a counterbody.
According to a further developed embodiment of a hollow-cylindrical and preferably tubular steel component, which extends along the portion with the larger inside diameter of the actuating piston, said steel component at its end directed towards the first portion of the actuating piston comprises a radially directed annular flange, which preferably projects radially inwards. Such a radial annular flange stiffens the region of the steel component in which the axial-force-transmitting conical surface is disposed.
In preferred embodiments of the actuating piston according to the invention, the steel component extends not only along the second portion of the actuating piston, which has the larger inside diameter, but also extends into the first portion of the actuating piston with the smaller inside diameter. When, for example, the steel component is provided with a radially inwardly projecting annular flange which additionally stiffens the region of the steel component in which the axial-force-transmitting conical surface is disposed, then the steel component preferably continues from said radial annular flange axially into the first portion with the smaller inside diameter of the actuating piston. In said case, in a particularly preferred manner the transitional region between the radial annular flange and the continuation of the steel component into the first portion of the actuating piston is rounded off in order to facilitate damage-free introduction of sensitive components such as, for example, a seal. The steel component may extend merely into the first portion of the actuating piston but it may also extend over the entire length of the first portion of the actuating piston.
According to a still further developed embodiment of the actuating piston according to the invention, the steel component comprising the axial-force-transmitting conical surface takes the form of a substantially pot-shaped insert, which is accommodated in the plastic or light metal part and forms a part of the end face of the actuating piston directed towards a brake disk. Given a slightly increased overall weight compared to the previously described embodiment, the last-mentioned embodiment has the advantage that, even in the case of hydraulic actuation, virtually all of the axial brake application forces are transmitted via the pot-shaped insert to the brake lining, with the result that load is removed from the mechanically less stable plastic or light metal part of the actuating piston according to the invention.
The steel component, which comprises the axial-force-transmitting conical surface, of an actuating piston according to the invention, when the component of the actuating piston cooperating with the cylinder bore of the disk brake is made of light metal, is preferably cast en bloc into the light metal or is extrusion-coated with the light metal. When the component of the actuating piston cooperating with the cylinder bore of the disk brake is made of plastics material, then the steel component is preferentially extrusion-coated with the plastics material or likewise cast en bloc into the plastics material. The extrusion-coating of the steel component or the casting of the steel component en bloc into the plastic or light metal component results in an extremely intimate and hence stable connection of said two components of the actuating piston according to the invention.
Depending on which plastics material is used for the plastic component, it may be necessary to anneal the plastic component in order to achieve the required strength. As many plastics materials shrink during an annealing process so that the dimensions of the plastic component alter, it may be advantageous for the plastic component to be separately manufactured and annealed before then being connected to the steel component. In such a case, the plastic component is preferably connected by glueing or pressing to the steel component to form a single-piece composite component.
Independently of whether the plastic component is made of a material which has to be subjected to an annealing process and possibly shrinks during said process, glueing of the steel component into the plastic or light metal component has the advantage that the adhesive layer may compensate differences in expansion which may exist between the steel component and the plastic or light metal component. A further advantage is that, when the preferably tubular steel component is glued into the prefabricated plastic or light metal component, the centre line of the steel component may easily be brought into alignment with the centre line of the plastic or light metal component because the adhesive layer may be utilized to compensate the manufacturing tolerances between the steel component and the plastic or light metal component.
The adhesive may be applied over the entire surface of the steel component and/or the plastic or light metal component, but it may alternatively be provided in the form of an epoxy resin ring which is inserted between the steel component and the plastic or light metal component. Upon subsequent heating of the two components up to around 150xc2x0 C., the epoxy resin melts and glues the plastic or light metal component in a hydraulically sealed manner to the steel component.
Furthermore, when the steel component is to be glued into the plastic or light metal component, the steel component together with the adjusting mechanism of the brake disposed therein may be preassembled into a constructional unit. Said constructional unit comprising adjusting mechanism and steel component is then glued into the plastic or light metal component. The temperature arising during the glueing-in process is so low that seals of the adjusting mechanism are not damaged.
The term, xe2x80x9ccomposite componentxe2x80x9d, used here also covers embodiments of an actuating piston according to the invention, in which there are minimal gaps between the plastic or light metal component and the steel component. In order, despite such gaps, to achieve a hydraulically sealed connection between the plastic or light metal component and the steel component, seals such as O-ring seals may be provided between said two components. Such embodiments of the actuating piston according to the invention likewise make it possible first to preassemble the steel component together with the adjusting mechanism of the brake to form a constructional unit and then to connect said constructional unit to the plastic or light metal component.
In order to increase the stability of the connection of the steel component to the plastic or light metal component even more, according to a modified embodiment of the actuating piston according to the invention the steel component at an outer circumferential surface is provided with projections, which effect positive locking between the steel component and the plastic or light metal component. Said projections may, for example, take the form of circumferential axial gearing. Other types of projections are however also suitable, e.g. an axial and circumferential knurl of the outer circumferential surface of the steel component.
In preferred refinements of the actuating piston according to the invention, the steel component at a side lying axially opposite the axial-force-transmitting conical surface comprises at least one generously dimensioned, radial annular surface. The stress on the material (in particular the compression of the material) of the light metal or plastic component which arises during the transmission of a high axial force at the transition between the steel part and the plastic or light metal part is thereby markedly reduced.
Besides the actuating piston according to the invention, the present invention also relates to a spot-type disk brake for motor vehicles, which is equipped with such an actuating piston and the weight of which is reduced by virtue of the use of the actuating piston according to the invention.